This invention relates to the production of extremely thin films of platinum on electrically conductive tin oxide surfaces. The resulting material may be used for many of the purposes for which platinum is used, including heterogeneous catalysts for gas phase and liquid phase reactions and electrodes for electro-chemical processes. Also, this invention contemplates producing a thin platinum film on optically transparent tin oxide surfaces, wherein the property of optical transparency is retained while imparting the electrochemical properties of platinum to the tin oxide surface.
A well established technology exists for the production of electrically conductive tin oxide coatings on glass, quartz, and other ceramic bodies. U.S. Pat. No. 2,564,707, issued to John M. Mochel on Aug. 21, 1951 describes the preparation of such coatings by spraying aqueous tin-containing solutions upon heated surfaces of the substrate. A different type of procedure, involving the gas-phase decomposition of tin compounds onto solid surfaces is described, for example, in The Electrochemical Society, 123, 941 (1976), B. J. Baliga and S. K. Ghandhi. Another recent work which deals with a vapor decomposition method of making tin oxide coatings is based on the reaction of stannic chloride (SnCl.sub.4) with oxygen and is described in The Journal of the Electrochemical Society, 125, 110 (1978). Extensive use of tin oxide-coated glass as electodes have been described in various literature articles. These uses include the study of electrode products by optical means, taking advantage of the transparency of such electrodes, as for example, J. W. Strojek and T. Kuwana, J. of Electroanalytical Chem. 16, 471 (1968). Other applications do not depend upon transparency, but take advantage of the low background currents and stability of such electrodes in acidic solutions at highly oxidizing potentials. (H. A. Laitinen, J. M. Conley, Anal. Chem., 48, 1224 (1976).
For many electrochemical applications, tin oxide electrodes are disadvantageous as compared with platinum because of high overpotentials, which result in prohibitive power losses in commercial processes. The use of a thin coating of platinum, in principle, makes it possible to extend the use of platinum of requiring much lighter loadings than are used at present. Examples of platinum-containing electrodes now in use include fuel cell membrane electrodes (as utilized, for example, by General Electric in the fuel cell power source in the early U.S. satellites), and platinized porous graphite electrodes used by Union Carbide in the aqueous, alkaline hydrogen-oxygen fuel cell.
Various methods have been developed in recent years which employ platinum metal and conductive tin oxide.
For example, U.S. Pat. Nos. issued to F. Beck, et al. (3,672,990), J. H. Entwisle (3,701,724) and C. R. Franks et. al. (3,875,043 and 3,917,518) relate to coatings which include physical mixtures of platinum metals and/or the oxides thereof with tin oxide. However, none of the patents is directed to the concept of placing a platinum metal layer onto a conductive tin oxide layer.
U.S. Pat. No. 3,674,675 to F. H. Leaman discloses a platinum-coated electrode having a palladium substrate rather than one of tin oxide. Tin is used in the patented process (column 2, line 41) but only to provide sites for the deposition of palladium, the tin being completely replaced by palladium during this process. Similarly, U.S. Pat. No. 3,711,385 to H. Beer concerns the formation of a thick platinum oxide layer on the surface of several metals, not including tin or tin oxide.
Other prior art includes U.S. Pat. Nos. issued to E. H. Cook Jr. (3,882,002, 3,940,323, 3,956,083 and 3,986,942) describing an electrode which has a tin oxide substrate doped with elements such as molybdenum, tellurium, or antimony, and a surface layer of a metal or metal oxide from the platinum group of metals. The primary differences between the Cook inventions and the present invention include: (1) Cook's outer coatings are noble metal oxides, while the present invention uses platinum only; (2) although the general description in Cook (e.s. U.S. Pat. No. 3,940,323, col. 5. lines 25-26) mentions electroplating as a possibility, the detailed description employs only chemical decompositon methods to apply the metal oxide coating to the substrate; and (3) Cook's range of noble metal oxide coverage is 0.1 g/m.sup.2 to 20 g/m.sup.2 with a preferred range of 3-10 g/m.sup.2 which is many times greater than that of the present invention. Further, the Cook patents fail to disclose whether the range of coverage measures the actual surface area or the projected area of the actual surface. The difference can be very large because the projected area fails to account for profile variations in the surface in the same way that an aerial photograph of mountainous terrain fails to show the actual surface area of the photographed terrain. It is the object of this invention to produce extremely thin films of platinum on electrically conductive tin oxide surfaces wherein the properties of the resulting product are rendered substantially similar to the properties of platinum itself.
Another object of the invention is to produce a thin platinum film on optically transparent tin oxide surfaces, wherein the property of optical transparency is retained while imparting the properties of platinum to the resulting product.
The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description.